Hemichordates are a group of marine animals that belong to the phylum Hemichordata. They are characterized by their body structure, which includes a proboscis, a collar, and a trunk. Hemichordates are primarily found in shallow waters and can be either free-swimming or burrowing. They play a role in the marine ecosystem by contributing to the sediment and nutrient cycles. There are two main classes of hemichordates: Enteropneusta, which are acorn worms, and Pterobranchia, which are small, colonial organisms. Hemichordates share some features with chordates, such as a notochord-like structure, indicating a close evolutionary relationship.